1. Field of the Invention
The present invention relates to a body structure of a motorcycle, in which a rear swing arm is mounted on a pivot portion of a crankcase.
2. Description of Background Art
Japanese Patent Laid-open No. Hei 1-306384 and Japanese Utility Model Laid-open No. Hei 1-170090 disclose body structures of motorcycles of a type in which a pivot portion is provided on a rear portion of a crankcase and a rear swing arm is directly mounted on the pivot portion.
Japanese Utility Model Laid-open No. Hei 1-57982 discloses a body structure of a motorcycle, in which a link is mounted on a rear portion of a crankcase and a rear swing arm is mounted on the link.
Japanese Patent Laid-open No. Hei 1-175590 discloses a body structure of a motorcycle, in which a pivot portion having an eccentric cam is provided on a vertically extending frame and a rear swing arm is mounted on the pivot portion.
Japanese Utility Model Publication No. Sho 57-35619 discloses a structure in which steps are directly mounted on side surfaces of a crankcase. Japanese Utility Model Laid-open No. Sho 61-10291 discloses a body structure of a motorcycle, in which step holders and a rear swing arm are co-fastened to a frame using a pivot shaft. The above-described document, Japanese Patent Laid-open No. Hei 1-306384 discloses a structure in which a side stand is mounted to a crankcase through a bracket.
Japanese Utility Model Publication No. Hei 6-49506 discloses a motorcycle in which a main frame extending rearwardly from a head pipe is formed into an eye-like shape in cross-section. A down-frame extends downwardly from a front portion of the main frame. A pivot frame extends downwardly from a rear end portion of the down-frame. An engine is supported by these frames and a rear swing arm 16 is rockably supported on the pivot frame.
The provision of a pivot portion on a crankcase is useful not only for improving the running performance of a motorcycle, but also for reducing the width of the body and reducing the weight of the body because of the provision of no pivot frame.
In each of the related art structures for the pivot portions, however, there occurs a problem that the assembly of a pivot portion requires a lot of labor in the process of assembling a body having such a structure because a crankcase seems to be divided right and left in accordance with a usual manner. Accordingly, it is expected to further improve the assembling performance of the pivot portion.
In the case where a pivot portion is directly mounted to a crankcase, a mounting portion thereof requires a high rigidity, so that a peripheral portion of the pivot portion must be enlarged or a separate member for supporting a rear swing arm must be provided.
Further, a mounting space for steps or the like requiring a sufficient mounting strength must be additionally ensured, and in the case where the steps or the like are mounted on the crankcase, there occurs the same problem as described above, that is, the mounting portions for the steps on the engine side must be enlarged.
In the case where a pivot frame is provided as in Japanese Utility Model Publication No. Hei 6-49506, since a twisting force applied from the rear wheel side to a pivot portion is dispersed and absorbed by a body frame such as a main frame, it is not so difficult to ensure the rigidity of the connection portions for connecting an engine to the body frame. However, in the case where such a pivot frame is not provided, it becomes very important to ensure the rigidity of the connecting portions for connecting the engine to the body frame.